In an induction motor, a rotor is mounted in a stator and is separated from the stator by an air gap. During operation of the motor, alternating currents supplied to windings of the stator induce currents in windings of the rotor. Due to magnetic material saturation, leakage inductance from the stator tends to be lower in regions in which rotor magnetic flux circulates than in regions in which rotor flux is not present. Because rotor magnetic flux direction is associated with a direct axis (d-axis), quadrature axis (q-axis) leakage inductance is lower than direct axis leakage inductance.
It is known to control the operation of an induction machine based on variation of rotor inductance. In at least one known method that has been proposed for use with pulse-width modulation (PWM) inverter-controlled motors, a fluctuating signal to the motor is injected in a synchronously rotating reference frame. The fluctuating signal is a high frequency voltage or current, for example, on the order of a few hundred Hertz, in addition to other voltages required by the motor for normal operation. An impedance difference between the flux axis and the orthogonal axis is then observed. The high-frequency voltage or current magnitude can be used as an error signal that can drive a PI (Proportional-Integral) controller that estimates flux angular velocity and position. If the high-frequency signal is injected in an estimated d-axis where the leakage inductance is at a minimum, the high-frequency current should be at a maximum. In an orthogonal axis, the high frequency current should be zero, corresponding to a region of maximum inductance.
For example, in one known controller configuration numbered generally as 20 in FIG. 1, a high frequency control signal is used as an error signal in estimating flux angular velocity and flux angle. Motor stator currents iα and iβ in a stationary reference frame are transformed at block 22 into an estimated flux reference frame, which is synchronously rotating with angular speed ωe. The current iqm has a DC component corresponding to the value of torque current and a high frequency component corresponding to injection voltage injected at a multiplier block 26. The injection voltage signal is useful for estimating ωe and θe, a stator flux angle used in vector control of the machine. In addition to these known components, iqm may contain a component at 6ωe due to imperfect dead-time compensation and a component at 2ωe due to unbalanced gain in the measurement of iα and iβ. A component at the stator frequency ωe could also be present if current sensor offset is not properly compensated.
To eliminate these unwanted components as well as the DC component from the injection component, there is provided a band-pass filter (BPF) 24 tuned at the injection frequency ωi. However, if BPF 24 is too selective (i.e., has a high quality factor Q), it can reduce the dynamic performance of the estimation block. In many configurations, then, a quality factor Q lower than 1 is used for dynamic reasons. Consequently, unwanted harmonics of iqm still can be introduced in the signal path.
An open loop configuration is generally used to eliminate such harmonic components. For example, harmonic components Â sin(6ωet+{circumflex over (φ)}6) and {circumflex over (B)} sin(2ωet+{circumflex over (φ)}2) are determined as further described below and are removed by adders 28 and 30. The resulting signal is averaged to an essentially DC component by a low-pass filter (LPF) 32, the result of which is passed through a proportional integrator (PI) controller or regulator 34. An estimate of flux angular speed ωe—est is added by an adder 36. This estimate is obtained from a vector control system (not shown) utilizing a slip angular speed estimate ωslip—est and a rotor speed estimate ωr—est. The result, ωe, is integrated by an integrator 38 to obtain a stator flux angle θe that is used in vector control of the machine.
The components Â sin(6ωet+{circumflex over (φ)}6) and {circumflex over (B)} sin(2ωet+{circumflex over (φ)}2)
obtained by carefully mapping amplitudes Â and {circumflex over (B)} as well as their phase shifts {circumflex over (φ)}6 and {circumflex over (φ)}2 as functions of the operating torque of the motor. This mapping can take considerable time. Moreover, harmonic components can change as a function of variables (such as motor temperature and inverter temperature) that are difficult to take into account. Thus, accuracy of the result can be adversely affected.